1. Field of the Invention
The present invention relates to a measurement apparatus and a method each using an electromagnetic wave. The present inventions also relates to tomography apparatus and method.
2. Description of the Related Art
Terahertz waves are typically electromagnetic waves containing components within a frequency band between 0.03 THz and 30 THz. Many characteristic absorptions originated from structures and states of various substances, such as biomolecules, are seen in such a frequency band. By using the characteristic, an inspection technique to non-destructively analyze or identify substances has been developed. Also, application to a safer imaging technique using, instead of X-rays, and application to a high-speed communication technique have been proposed. Further, application to a tomography apparatus which visualizes inside of an object by using, for example, terahertz waves reflected by a refractive index interface inside the object, has been attracting an attention. By using this apparatus, it is expected that the structure inside the object can be visualized at the depth of about several 100 μm to about several 10 mm by taking advantage of the penetrating characteristic of terahertz waves.
Concerning the above-described techniques, Japanese Patent No. 4,046,158 discloses an example which measures thicknesses of an object based on a time interval between a plurality of terahertz wave pulses reflected by a plurality of interfaces of the object.
In the case of an apparatus which uses a time interval between a plurality of terahertz wave pulses, the accuracy of the measured time interval between the terahertz wave pulses is important. It has so far been considered that the time interval between terahertz wave pulses is greatly dependent on the interval between the interfaces of the object. However, according to the investigation of the present inventors, it is found that, in an apparatus configuration in which a terahertz wave pulse is collected and irradiated to an object, the time interval between terahertz wave pulses is also changed in dependence upon the position of the object arranged in the propagation path of the terahertz wave. Specifically, it was found that, for example, when one interface is located in a region in which the terahertz wave pulse propagates in a collimated manner (and which is also referred to as a collimated propagation region in this specification and which wave-optically corresponds to the depth of focus), and the other interface is located in the region in which the terahertz wave pulse is in a process of being collected (and which is also referred to as a collecting process region in this specification), a difference is caused between the optical paths along which the terahertz waves respectively propagate from the interfaces, and this difference in the optical paths is superimposed on the time interval.
Japanese Patent No. 4,046,158 does not show detailed descriptions cocnering an adjustment of the relative positional relationship of the plurality of interfaces of the object with respect to the optical system (also referred to as a convergence unit in this specification) for collecting a terahertz wave to the object. Therefore, a measurement result which is different from the interval between the actual interfaces may be output depending on the relative positional relationship between the object and the convergence unit. That is, the reliability of measurement accuracy may be lowered. Also, in a measurement method in which an electromagnetic wave irradiated to the object is continuous wave (CW), despite that a the used conception is not the same as described in Japanese Patent No. 4,046,158, the possibility of lowering the reliability of measurement accuracy can be pointed out.